The present invention relates to a process for the preparation of adipic acid and other lower aliphatic dibasic acids by the oxidation of saturated cyclic hydrocarbons.
Adipic acid is a major article of commerce and its preparation has, therefore, attracted much attention. Consequently, many processes for the production of adipic acid have been proposed. For example, one process involves nitric acid oxidation of cyclohexanol, cyclohexanone, or mixtures thereof, which can in turn be obtained by air oxidation of cyclohexane or hydrogenation of phenol. Several of these known processes are practiced commercially, but all suffer from high costs associated with such multi-step operations and the use of nitric acid, as well as from significant environmental pollution problems caused by the discharge of ozone-depleting nitrogen oxide by-products generated during nitric acid oxidation.
Processes that have been proposed for preparing dibasic acids without the use of nitric acid include air oxidation of saturated cyclic hydrocarbons and/or corresponding cyclic ketones and/or alcohols. For example, U.S. Pat. No. 3,390,174 and British Patent 1,304,855 disclose processes requiring mixtures of two or more of these components. However, many air oxidation processes are multi-step processes having poor selectivities and requiring difficult high-cost recovery processes. Nevertheless, an air oxidation process that provides good yields of a single dibasic acid free of significant by-products would be highly desirable.
Catalytic air oxidation processes are believed to involve free radical oxidation. Such oxidations are complex systems in which many types of reactions other than oxidation can occur. Free radicals will attack any C--H bonds, regardless of form, to an extent determined by bond strength and relative concentration of the specific C--H bond. As oxidation proceeds, various oxygenated compounds form, such as alcohols, aldehydes, ketones, and acids (including difunctional compounds having these functionalities), as well as other low molecular weight carbon compounds. All of these compounds can further react via acid catalysis or thermal ionic mechanisms to form various condensation products, the most prevalent being esters. In general, the amount of condensation products will increase as the rate of oxidation relative to ester formation decreases. Process modifications that improve the ratio of oxidation to ester formation would be expected to yield a greater amount of easily recoverable diacid. In addition, modifications that lower the rate of esterification would be expected to improve the amount of easily recoverable diacid.
Until now, however, the seemingly attractive direct oxidation routes have not provided a viable commercial process, possibly because of the complexity of the reaction residues ("bottoms") containing many different simple esters derived from the various intermediates, oxidation products, and post-oxidation products. Such complexity is not unique to saturated cyclic alkane oxidations. These complex reactions exist even for oxidations of aromatic compounds such as xylenes. The primary distinction is that bottoms from methyl-substituted aromatic oxidation (i.e., intermediates, derivatives, and the like) can be subjected to very stringent oxidation conditions that allow continued oxidation to oxidation-stable aromatic acids. For example, the aromatic acid products are extremely stable to further oxidation and can be subjected to extreme conditions under which a substantial amount of the seemingly inert acetic acid would be oxidized to CO.sub.2 and water. Consequently, these aromatic acid products can be produced substantially free of oxidation bottoms, intermediates, derivatives, and the like at very high conversions of 95% or higher.
Aliphatic diacids such as adipic acid, on the other hand, are subject to further oxidation because the C--H bonds of the methylene groups in such acids can more readily undergo free radical attack and oxidation. If subjected to forcing oxidation conditions at higher conversion, the various bottoms, intermediates, and derivatives will oxidize (as do similar aromatic compounds). However, because of the relative instability to oxidation of the aliphatic acids (such as adipic, glutaric, and succinic acids, and even acetic acid under stringent conditions), these acid products will progressively and increasingly degrade to CO.sub.2 and water, thereby providing lower selectivity.
It was, therefore, an object of the present invention to avoid these problems and achieve a chemical process having desirable commercial features.
Single-step direct air oxidation processes for the production of dibasic acids have also been proposed. However, previous one-step processes have been attended with poor selectivity, low production rate, multi-step operation, burdensome and costly separation, and low ultimate overall yields of dibasic acids from the saturated cyclic hydrocarbon. For example, U.S. Pat. No. 2,223,493 discloses a process for the direct oxidation of cyclohexane to form adipic acid at a reported production rate of 3.1 wt. % per hour in a concentration of 12.4 wt. % in the oxidation effluent with an overall selectivity of 46 to 49 mole %. This oxidation was carried out using a comparatively high concentration of cyclohexane (about 61 to 63 wt. %) in acetic acid solvent in the presence of air and various catalysts at temperatures of from 95.degree. C. to 120.degree. C. until a conversion level of about 23 to 24% was achieved.
U.S. Pat. No. 2,589,648 discloses a single-step oxidation process in which acetone is used instead of acetic acid as solvent.
U.S. Pat. No. 3,231,608 discloses another single-step direct oxidation process for the production of dibasic aliphatic acids. The reference teaches that the use of certain critical ratios of solvent and catalyst to the saturated cyclic hydrocarbon can yield dibasic aliphatic acids under mild reaction conditions, usually at production rates of adipic acid of 3.5 to 4.0 wt. % per hour and at efficiencies generally around 73 to 76 wt. %. In particular, the reference teaches that molar ratios of solvent to saturated cyclic hydrocarbon in the range of 1.5:1 to 7:1 (or more) are suitable but that molar ratios below or above this range give unsatisfactory results. A comparison example carried out using molar ratios more nearly like those of the present invention gave a decidedly inferior adipic acid production rate. The process of the present invention provides excellent results despite using molar ratios of solvent to cycloaliphatic hydrocarbon well below the range specified for the process disclosed in U.S. Pat. No. 3,231,608.
Additional references describe attempts to improve upon the process of U.S. Pat. No. 3,231,608. A general objective of these references was attainment of higher conversions of cyclohexane, which was usually achieved by lowering the starting concentration of cyclohexane, by using protracted reaction times, or by making other such changes, with the result being very low reaction rates, reduced selectivities, and expensive recovery and downstream processing. For example, U.S. Pat. Nos. 4,032,569 and 4,263,453 require a greater relative amount of cobalt(III) catalyst (and U.S. Pat. No. 4,263,453 requires small amounts of water) but still specify essentially the same molar ratios of solvent to cycloalkane as U.S. Pat. No. 3,231,608. G. N. Kulsrestha et al in Chem. Tech. Biotechnol., 50, 57-65 (1991), similarly discloses an oxidation process that uses a relatively large excess of acetic acid and a large amount of cobalt(III) catalyst. U.S. Pat. No. 4,158,739 discloses a similar preparation of glutaric acid from cyclopentane in which the molar ratio of solvent to cyclopentane must be at least 1.5:1 and the amount of catalyst is relatively higher than for U.S. Pat. No. 3,231,608. In general, the use of excess acetic acid solvent at the higher molar ratios disclosed in the prior art appears to reduce the rate of adipic acid product.
Further details on a known single-stage oxidation process for the preparation of adipic acid from cyclohexane are discussed by K. Tanaka in Chemtech, 555-559 (1974), and Hydrocarbon Processing, 53, 114-120 (1974).
It has now surprisingly been found that, contrary to the expectations of the prior art, the oxidation of high concentrations of cycloalkanes at low conversion levels provides advantageous chemical and economic results. For example, use of high cyclohexane concentrations at restricted conversion levels, in conjunction with mild reaction conditions and with catalysts such as cobalt(II) or cobalt(III) ion that generate free radicals in an organic acid/or mixed solvent, gives a rapid rate of adipic acid production with minimal structural loss of C.sub.6 compounds (that is, with lower conversion to C.sub.5, C.sub.4, or lower of carbon-containing by-products). The oxidation process of the invention permits a surprisingly facile recovery of adipic acid because of the strong tendency of the oxidation effluent upon cooling to separate cleanly into phases. The large non-polar upper phase can be directly recycled for oxidation without costly processing, whereas the polar lower phase is extremely rich in adipic acid that can be recovered in high yield by filtration or centrifugation, with the filtrate or supernatant can to a large degree be directly returned to oxidation without costly reprocessing.